1. Field of the Invention
The present invention generally relates to a shift register and a shift register apparatus thereof, and more particularly, to a shift register for avoiding an output voltage incapable of being completely charged due to a bias applied to a gate of an amorphous silicon thin film transistor (a-Si TFT) for a long time and a shift register apparatus thereof
2. Description of Related Art
In a condition of a predetermined panel processing, it is critical for configuring a circuit on a glass substrate to be used an a-Si TFT. Generally, a-Si TFTs can be divided into two types, P-type a-Si TFTs and N-type a-Si TFTs. The P-type a-Si TFT typically has a threshold voltage higher than that of an N-type a-Si TFT. Therefore, P-type a-Si TFTs are often considered as less convenient than N-type a-Si TFTs. As such, all N-type a-Si TFTs are more often to be formed when configuring a circuit on the glass substrate.
An all N-type a-Si TFT includes an inverter which is usually configured on the glass substrate. FIG. 1 is circuit of an inverter of a conventional all N-type a-Si TFT. Referring to FIG. 1, the circuit includes transistors 101 and 102, wherein VDD and GND represent a power source voltage and a ground voltage, respectively, and Vin and Vo represent input signals and output signals, respectively.
It can be learnt from FIG. 1, that a gate of the transistor 101 is directly coupled to the power source voltage VDD. However, such a bias configuration negatively affects the inverter circuit, in that if the gate of the a-Si TFT is applied with a direct current bias for a relative long time, the a-Si film will be degraded and incur many defects. In the meantime, a threshold voltage Vt thereof inevitably is significant. In this manner, in the circuit structure illustrated in FIG. 1, the threshold voltage Vt of the transistor 101 will shifts after a certain time of operation, so that a voltage of the output signals Vo can not be completely charged. Therefore, the inverter circuit is not suitable for long time operation.
Currently, a-Si TFTs have been used for fabricating shift register apparatus on glass substrates. Some technologies disclosed in related patents, such as U.S. Pat. Nos. 7,038,653, and 5,222,082, bypass the inverter structure as shown in FIG. 1 by sophisticated design. However, the problem of the shift of the threshold voltage of the a-Si TFT occurred after long time use has not yet been resolved. FIG. 2 is a circuit of a shift register apparatus disclosed in U.S. Pat. No. 7,038,653. As shown in FIG. 2, a transistor 201 of the inverter structure defined in the dashed frame 174 is being continuously applied with a bias voltage of VON, so that the transistor 201 would also have a problem of shift of the threshold voltage. This problem drastically shortens a lifetime of the shift register apparatus.
FIG. 3 shows a circuit of a shift register apparatus disclosed in U.S. Pat. No. 5,222,082. As shown in FIG. 3, there is no circuit structure similar to what is shown in FIG. 1. However, when the shift register apparatus is operated, residual charges on a node P2 cannot be discharged therefrom, and a transistor 301 also suffers a bias voltage for a relative long time. After the shift register apparatus is being operated for an excessively long time, a threshold voltage of the transistor 301 increases drastically, so that a voltage at an output terminal OUTPUT can not be completely charged, and thus the shift register apparatus may be out of use for quite a long time.
Several manufacturers have attempted to solve the problem of the shift of threshold voltage after a long time operation of a-Si TFTs. However, no effective solution has been found so far.